A fundamental technology employed in the field of molecular biology is coupling of nucleic acids. The coupling of nucleic acids is used, for example, in combination with hybridization, for the introduction of genes or detection of base sequences. Therefore, coupling of nucleic acids is an extremely important technology used not only in the basic research in molecular biology, but also in, for example, the diagnosis or treatment in the field of medicine, the development or production of therapeutic drugs, diagnostic drugs or the like, or the development or production of enzymes, microorganisms or the like in industrial and agricultural fields.
Coupling of nucleic acids has conventionally been carried out using, for example, DNA ligases and the like. However, reactions carried out by taking out such in vivo enzymatic reactions must be carried out under specially set conditions, and the reactions have disadvantages such as relatively high prices of the enzymes used and insufficient stability. In order to overcome such disadvantages, research has been conducted to find technologies for the coupling of nucleic acids which do not make use of enzymes.
As one such technology for the coupling of nucleic acids that does not use enzymes, there is an available method of using an organic compound which is capable of reacting with a nucleic acid. In recent years, technologies for the coupling of nucleic acids utilizing photoreaction are increasingly attracting attention because of their advantages such as that the temporal and spatial control of the reaction is freely achieved, and the reaction can be carried out under milder conditions as compared with general organic chemical reactions.
Among photocoupling technologies as such, those photocoupling technologies making use of 5-cyanovinyldeoxyuridine (Patent Document 1: Japanese Patent No. 3753938 and Patent Document 2: Japanese Patent No. 3753942) are known.
Another important technology similar to the coupling of nucleic acids is crosslinking of nucleic acids. For example, a crosslinking reaction of DNA or RNA blocks the intracellular flow of genetic information, and thus is used in the inhibition of gene expression. As a crosslinking agent for DNA, a compound called psoralen has been traditionally used as a photo-crosslinking agent (photo-crosslinking agent) that induces crosslinking through a photoreaction. A therapeutic method of internally taking psoralen, which is a photo-crosslinking agent, as a medicine, and carrying out photoirradiation, is in widespread use as one of standard therapeutic methods for psoriasis, which is a skin disease.
However, since the photo-crosslinking reaction of psoralen occurs preferentially with a 5′-TA-3′ sequence between the two strands of nucleic acid, there is a problem that the target of the reaction that can be used is restricted. Furthermore, since the photocoupling wavelength for psoralen is 350 nm while its photocleavage wavelength is 250 nm, it is needed to use a short wavelength light source, in the case of using psoralen as a photo-crosslinking agent. Therefore, there is a problem that photoirradiation is likely to cause damages to DNA or cells.    Patent Document 1: Japanese Patent No. 3753938    Patent Document 2: Japanese Patent No. 3753942